Economy integration is now handled through Vault instead of Register. You will need to download and install the Vault plugin now if you want to use any Economy-related features. If you're not using the Register plugin for anything other than Factions, it should be safe to remove it from your server.

Vault: http://dev.bukkit.org/server-mods/vault/

Note: for proper faction bank support in iCo5 and EssentialsEco, I recommend waiting for Vault 1.2.5 to be released which addresses faction account creation issues.

src/com/massivecraft/factions/iface/EconomyParticipator.java

@@ -1,10 +1,8 @@
 package com.massivecraft.factions.iface;
 
-import com.nijikokun.register.payment.Method.MethodAccount;
-
 public interface EconomyParticipator extends RelationParticipator
 {
-	public MethodAccount getAccount();
+	public String getAccountId();
 	
 	public void msg(String str, Object... args);
 }